In this project we will seek to resolve the origin of differences in tonic and phasic response output from the hair cell and cupula of the vestibular system. Specifically, we will determine if differences in encoded vestibular output is attributable to: (1) cupula motion and regional variation in hair cell stereociliary deflection; (2) afferents that are differentially (numerically and/or regionally) innervated by hair cells; and/or (3) that afferents may respond differentially to a transmitter quantum. This will be accomplished through a coordinated interdisciplinary effort encompassing simulation studies described throughout this program project application. Silver's laboratory will focus upon the acquisition and analysis of images of sterociliary and cupular movements during head displacement, by developing a digital video light microscopic method for directly observation, analysis and relating movements of sub-cellular, cellular and supra-cellular structures in the vestibular end organ (VEO) related with vestibular function, using the toadfish (Opsanus tau) labyrinth as the model system, Reeves' group will focus upon advanced analytical imaging methods. High quality imagery of the performance of VEO components is needed to establish the structure function relationships of these essential mechano-electrical transducers, especially in light of the advances made by the collaborating laboratories in hair cell electrophysiology and computational modeling. In both cases, detailed information about the actual motions and fine mechanical displacements of these transducers obtained on-line during and actual experiment is lacking. Recently, Silver, in collaboration with Highstein, accomplished direct video light microscopic imaging of individual stereocilia in tact labyrinths. To achieve our project goals we will: (1) develop a light microscope capable of video observation and recording of sterociliary and cupular movements in situ in concert with concurrent electrophysiological recordings; (2) describe the sub- micrometer displacements of these structures during VEO movements via computational representations of image data (i.e., processed imagery, isosurfaces and isovolumes) to aid in testing for the parity correspondence among actual VEO movements (this work), electrophysiological performance and kinetic simulations. Integration of the information developed will facilitate assessment of normal and dysfunctional VEO performance in normal, and microgravity environments.